This invention relates to a commutation control device for a current type inverter including a forced commutation circuit, which is adapted to stably drive an AC motor or the like.
The current type inverter is an inverter wherein a commutation energy in a load such as an electric motor is stored in a capacitor, and the stored energy is used for the subsequent commutation.
An example of a conventional current type inverter is illustrated in FIG. 1.
In this example, an electric current supplied from a variable DC source 1 and smoothed out by a DC reactor 2 is converted by a main thyristor bridge 3 comprising thyristors 21-26 into three phase currents which are supplied to a load 4 such as an AC motor. The example further includes an auxiliary thyristor bridge 5 comprising thyristors 51 through 56, additional thyristors 11 and 12, choppers 13 and 14, and diodes 15 and 16 which connect a capacitor 17 across the DC side of the auxiliary thyristor bridge 5 when the choppers 13 and 14 are turned OFF. The variable DC power source 1 may be of a type including a phase-controlled rectifying bridge connected to a three-phase AC power source. The choppers 13 and 14 may be made of gate turn off thyristors, transistors, or thyristors.
The operation of this conventional inverter will be described with reference to FIGS. 2, 3(a), 3(b) and 3(c).
Assuming that the thyristors 21 and 26 are ON at a time instant t.sub.0, electric currents I.sub.U and I.sub.W flow through the motor 4. When a commutation timing pulse P.sub.t is generated at an instant t.sub.1, the thyristor 11, choppers 13 and 14, and a thyristor 54 in the auxiliary thyristor bridge 5 are all turned ON. Thus, a reverse voltage is applied to the thyristor 21 to turn OFF the same. The current I.sub.U now changes its passage from the thyristor 21 to the thyristor 54 as shown in FIG. 3(a). During the interval t.sub.1 -t.sub.2, an electric current flows from the DC power source 1 through a loop comprising the reactor 2, thyristor 11, chopper 13, capacitor 17, chopper 14, thyristor 54, electric motor 4, and the thyristor 26, thereby discharging the capacitor 17.
When the choppers 13 and 14 are turned OFF and the thyristor 22 is turned ON at the end of the interval t.sub.1 -t.sub.2 of a sufficient length for turning OFF the thyristor 21, the electric current I.sub.U attenuates because its direction is opposite to that of the voltage V.sub.C across the capacitor 17, while the current I.sub.V increases as shown in FIG. 3(b) because it flows directly from the power source 1. The commutation, that is a transfer operation from the current I.sub.U to the current I.sub.V, completes as shown in FIG. 3(c) at an instant t.sub.3.
The voltage V.sub.C of the capacitor 17 decreases during the interval t.sub.1 -t.sub.2 due to the discharge of the capacitor 17, and increases during the interval t.sub.2 -t.sub.3 due to the charging of the capacitor. When the voltage V.sub.C becomes high, the variation rates of the currents I.sub.U I.sub.V increase, thus reducing the interval t.sub.2 -t.sub.3 so as to reduce the increasing rate of the voltage V.sub.C.
The rate of decreasing the voltage V.sub.C during the interval t.sub.1 -t.sub.2 is constant for a constant load current. Thus, when the interval t.sub.1 -t.sub.2 is constant, the voltage V.sub.C is brought into a constant value at the instant t.sub.2.
However, when the load condition varies, the voltage V.sub.C at the instant t.sub.2 also varies from the constant value. For instance, when the load increases, the voltage V.sub.C also increases which is applied to the motor 4 at the time of commutation thus increasing surge voltage. Conversely, when the voltage V.sub.C decreases below the back electromotive force of the electric motor 4, the current I.sub.U in FIG. 3(b) will not recuce to zero, thus resulting in a commutation failure.